值未插入到Hashmap中

Question

public class FileReaderServer implements Runnable
{
    private Map<String, String> mapp = new HashMap<String, String>();

    public void searchWords()
    {
        try
        {
            BufferedReader br = new BufferedReader(new FileReader(input));
            while ( (line = br.readLine()) != null )
            {
                boolean success = tree.search(line);
                if ( success )
                    mapp.put(line, "TRUE");
                else
                    mapp.put(line, "FALSE");
            }
        }
        catch( Exception ex )
        {
            System.out.println( ex );
        }
    }

    public void run()
    {
        searchWords();
    }

    public void printMap()
    {
        Iterator it = mapp.entrySet().iterator();
        while ( it.hasNext() ) 
        {
            Map.Entry pair = (Map.Entry)it.next();
            System.out.println(pair.getKey() + " = " + pair.getValue());
        }
    }

    public static void main( String[] args )
    {
        try
        {
            FileReaderServer server = new FileReaderServer("keywords.txt");
            server.createTree();

            Thread t1 = new Thread(new FileReaderServer());
            Thread t2 = new Thread(new FileReaderServer());
            Thread t3 = new Thread(new FileReaderServer());             

            t1.start();
            t2.start();
            t3.start();

            t1.join();
            t2.join();
            t3.join();

            server.printMap();
        }
        catch( Exception ex )
        {
            System.out.println(ex);
        }
    }

This is cut-down version of my original class enough to figure out issue I believe. ( if not please let me know to paste some more code.)

Issue is that in "printMap" nothing is getting printed. However, code has reached

boolean success = tree.search(line);
if ( success )
{
    System.out.println(line + "TRUE");   // This is getting printed.
    mapp.put(line, "TRUE");
}
else
    mapp.put(line, "FALSE");

EDIT :- One more question here. I'm trying to make searchWords synchronized . However, I am passing different instances to each of the created thread.So, is synchronization a property of object OR a class. ie whether any of these threads could be executing that function concurrently.

Answer 1

You pass another instance of FileReaderServer to each thread:

        Thread t1 = new Thread(new FileReaderServer());
        Thread t2 = new Thread(new FileReaderServer());
        Thread t3 = new Thread(new FileReaderServer());   

But you try to get values from:

        FileReaderServer server = new FileReaderServer("keywords.txt");

...

        server.printMap();

If you will pass server instance to each of these three threads - you will get unexpected behaviour due unsynchronized nature of java.util.HashMap . Replace it with java.util.concurrent.ConcurrentHashMap .

UPD1 If you will mark searchWords() as synchronized - you will loose all benefits of concurrency, because all your 3 threads will execute searchWords() sequentially.

So I'm still insist on using ConcurrentHashMap :

private Map<String, String> mapp = new ConcurrentHashMap<String, String>();

Or you may simply wrap HashMap instance with Collections.synchronizedMap(...) like this:

private Map<String, String> mapp = Collections.synchronizedMap(new HashMap<String, String>())

Answer 2

Your problem is, that you try to get your lines from server . But the server object will not get started. You have to start your threads like so:

Thread t1 = new Thread(server);
Thread t2 = new Thread(server);
Thread t3 = new Thread(server);   

In this case you have to think about good synchronization.

Answer 3

You are printing the HashMap member of the FileReaderServer instance referred by the server variable, but that's not the HashMap you are adding entries to.

You are adding the entries to the HashMap s of the FileReaderServer instances passed to the Thread s you are creating.

You may want to make mapp static if you want it to be shared by all instances of FileReaderServer . However, if you do that, you may encounter synchronization issues, so perhaps you should use a ConcurrentHashMap .

Answer 4

Where is the Constructor that takes a String File-name. Create a constructor that takes a String file-name as parameter.

FileReaderServer server = new FileReaderServer("keywords.txt");

Where are you initializing the variable input in:

BufferedReader br = new BufferedReader(new FileReader(input));

When it creates a thread FileReaderServer does not know which file to read.

Thread t1 = new Thread(new FileReaderServer());